Azide as a preservative in assays of aspartate aminotransferase activity.

Azide at an effective antimicrobial concentration, 7.7 mmol/liter, inhibits neither the cytoplasmic or the mitochondrial isoenzymes of human aspartate aminotransferase (EC 2.6. 1. 1) or porcine malate dehydrogenase (EC 1.1.1.37). It markedly prolongs the usefulness of substrates in assays for estimating aspartate aminotransferase activity (aspartate substrate without azide deteriorates in less than 24 h at 25 #{176}C) and does not atfect results obtained in assays in which activity is continuously monitored. It also has no effect on chromophore development or absorption spectra in 2,4-dinitrophenyihydrazine-coupled assays, but it eliminates oxalacetate-diazonium salt chromophore formation, thus making it unsuitable for use in this assay. At concentrations greater than 100 mmol/liter it inhibits activities of both enzymes. Azide also catalyzes the ketoenol tautomerization of oxalacetate.